Login geeft telkens fout

Login telkens error

<?php
if (!isset($_POST['submit'])){
?>
<!-- The HTML login form -->
	<form action="<?=$_SERVER['PHP_SELF']?>" method="post">
		Username: <input type="text" name="username" /><br />
		Password: <input type="password" name="password" /><br />
 
		<input type="submit" name="submit" value="Login" />
	</form>
<?php
} else {
	require_once("includes/config.php");
	
 
	$username = trim($_POST['username']);
	$password = trim($_POST['password']);

	$sql = "SELECT user_id from users WHERE username LIKE '{$username}' AND password LIKE '{$password}' LIMIT 1";
	$result = $db->query($sql);
	if (!$result->num_rows == 1) {
		echo "<p>Invalid username/password combination</p>";
	} else {
		echo "<p>Logged in successfully</p>";
		// do stuffs
	}
}
?>	

Ik weet zeker dat ik de juiste gegevens invoer. Krijg telkens
Invalid username/password combination

--

Dit is de database

CREATE TABLE `users` (
`user_id` int(11) NOT NULL,
`role` varchar(10) NOT NULL,
`username` varchar(50) NOT NULL,
`password` varchar(256) NOT NULL,
`email` varchar(256) NOT NULL
) ENGINE=MyISAM

Iemand enig idee waarom ik niet verder kom?